1. Solar and Stellar Flares Hugh S. Hudson SSL, UC Berkeley 1 May 20111

2. Outline Background Basic physics New developments Outstanding problems 1 May 20112

3. Three kinds of flares 1 May 20113 Stellar (Kowalski et al. 2010) Solar (Woods et al. 2004) Some planet (HST)

4. 1 May 20114 Impulsive phase and gradual phase of a solar flare Impulsive phase – primary energy release hard X-rays (10s of keV) white light, UV,  waves - broad spectrum duration < few minutes intermittent and bursty time profile, 100ms largely from the chromosphere Gradual phase - response to input thermal emission (kT ~0.1-1 keV) rise time ~ minutes coronal reservoir at work Impulsive phase: > few tenths of the total flare energy released (up to 10 32 ergs) Significant role for non-thermal electrons CME acceleration

5. The solar astronomer’s view 1 May 20115 A 1D model with no flows or plasma physics Full RT analysis links each emission to a height range cf. the new Asplund et al. abundances, justifying better models

6. 1 May 20116

7. 7

8. 8

9. 9

10. AIA on SDO 1 May 201110

11. Outline Background Basic physics New developments Outstanding problems 1 May 201111

12. Basic Physics The transition between the low-beta corona and the high-beta photosphere Radiative transfer structurally important Most flare theory is in the MHD framework RHESSI and particle acceleration 1 May 201112

13. Coronal view of a flare 1 May 201113

14. 1 May 201114

15. Chromospheric view of a flare 1 May 201115 Inverted colors showing continuum emission from the chromosphere in a 32 arc s domain From other data, we understand that this is the main flare luminosity It is very intense, and mostly unresolved in space and time

16. The world of cartoons http://solarmuri.ssl.berkeley.edu/~hhudson/cartoons/ 1 May 201116

17. The world of cartoons http://solarmuri.ssl.berkeley.edu/~hhudson/cartoons/ 1 May 201117

18. Four environments In a wind: Dungey Magnetar: Duncan In a static structure: Gold-Hoyle With disk: Shu 1 May 201118

19. The flare problem How does one extract energy suddenly from a stressed low-beta plasma? - In solar flares, we see hard X-rays resulting from ~30 keV electrons that appear to contain most of the converted energy - We also, in major events, see  -rays implying comparable energy in accelerated ions - In events with coronal mass ejections, the interplanetary particle acceleration can have 10% of the flare energy (Mewaldt), hence collisionless shocks are also important 1 May 201119

20. Outline Background Basic physics New developments – two paradigms in doubt Outstanding problems 1 May 201120

21. New Tools RHESSI – hard X-ray and  -ray imaging STEREO – true stereoscopic views of solar plasma SDO – comprehensive imaging and Sun-as-a-star spectroscopy through the EUV 1 May 201121 New Insights The chromosphere is after all the source of the main physics of a flare Stellar flares provide excellent tests of the ideas Concepts from the terrestrial aurora are important

22. Stellar flares 1 May 201122 YZ Cmi flare (Kowalski et al. 2010) Optical spectroscopy suggests that “a small A0 star appears briefly in the M star atmosphere” Also powerful Balmer lines and continuum; an optically thin source Solar spectroscopy is not this good II Peg flare (Osten et al. 2007) Hard X-ray signature captured Link to solar-type mechanisms

23. Stellar flares 1 May 201123 YZ Cmi flare (Kowalski et al. 2010) Optical spectroscpy suggests that “a small A0 star appears briefly in the M star atmosphere” Also powerful Balmer lines and continuum Solar spectroscopy is not this good II Peg flare (Osten et al. 2007) Hard X-ray signature captured Link to solar-type mechanisms

24. Auroral physics 1 May 201124 The coronal magnetic energy, at low plasma beta, must be released as field distortions (waves) The Alfvenic Poynting flux is therefore a likely transport mechanism (Fletcher & Hudson 2008) The chromosphere is a likely place for flux transfer (Haerendel 2011)

25. Alfven waves or particle beams? 1 May 201125 Properties of the low corona above a sunspot (ref. Allen’s Astrophysical Quantities) At 10 Mm, find 10 3 G, V A =c/3, and beta < 10 -5 Alfvén speed and plasma beta are similar to values in the geotail, but B is much larger

26. Paradigms at peril 1 May 201126 1.The thick-target model - Energy too intense (Krucker et al. 2011) - No hard X-ray directivity - No hard X-ray polarization 2.The standard reconnection scenario - seldom seen - no chromosphere - stability problem chromosphere electron beam Kopp-Pneuman 1976

27. Hard X-ray and  -ray imaging 1 May 201127 Flare SOL2003-10-28, TRACE background image/ RHESSI contours Hard X-rays (red contours) do not match  -rays (blue contours) (Hurford et al. 2006) These phenomena correlate in occurrence (Shih et al 2009) No reasonable mechanisms yet proposed

28. Outline Background – the solar atmosphere Basic physics New developments Outstanding problems 1 May 201128

29. Outstanding problems Formation of small A0 star (Kowalski) Acceleration of high-energy ions and electrons Unexplained 511 keV line width * Sunquake excitation * * sorry, no time to discuss... 1 May 201129

30. Conclusions The physics of solar and stellar flares has strong commonalities We should study the terrestrial aurora to learn about the microphysics NuSTAR will solve some solar problems * * sorry, no time to discuss them here 1 May 201130